Process for making polycarboxylic acid derived from polysaccharide composed of anhydrous glucose units and salts thereof

ABSTRACT

There is described a polycarboxylic acid or a salt thereof obtained by oxidation of a polysaccharide containing an anhydrous glucose as a constituent unit, wherein the carboxyl group content thereof is high and the weight average molecular weight thereof is not less than 2000. This polycarboxylic acid is suitable for use as a builder for detergent because the amount of an alkali required to perform neutralization titration of the polycarboxylic acid is not less than 435 mg in terms of sodium hydroxide and because the chelate force thereof to a polyvalent cation is high. 
     The polycarboxylic acid is produced by oxidizing a polysaccharide with an oxidizing agent in the presence of a transition metal catalyst.

This application is a 371 of PCT/JP96/01809, filed June 28, 1996, nowabandoned.

TECHNICAL FIELD

The present invention relates to a polycarboxylic acid or a salt thereofproduced by oxidation of a polysaccharide containing an anhydrousglucose as a constituent unit and to a method of producing same.

BACKGROUND ART

Polycarboxylic acids such as polyacrylic acid, copolymer of acrylic acidand maleic acid, etc. are used as is a dispersant, a chelating agent ora flocculant. It is generally known that, when these polycarboxylicacids are used as the chelating agent, the effect is influenced by thecontent of carboxyl groups and molecular weight and, furthermore, thelarger the content of carboxyl groups and molecular weight, the betterthe effect to be exerted. When a chelating capacity of thesepolycarboxylic acids is measured by the method shown in the Examplesdescribed hereinafter, the polyacrylic acid exhibits a Ca ionsequestration capacity of about 300 mg/g. The copolymer of acrylic acidand maleic acid exhibits a Ca ion sequestration capacity of about 370mg/g.

However, it is also well known that it is very hard to performbiodegradation of the polycarboxylic acid obtained by polymerizing amonomer having a vinyl group, such as polyacrylic acid, copolymer ofacrylic acid and maleic acid, etc., using microorganisms. Therefore, forthe purpose of obtaining a chelating agent which is useful as a builderfor detergent, there has been made a trial of oxidizing polysaccharidesas a natural polymer to obtain a polycarboxylic acid whosebiodegradation is expected, or a salt thereof, heretofore.

JP-B-49-1281 discloses that a polycarboxylic acid is obtained fromvarious polysaccharides by a two-stage oxidation method using periodicacid and chlorite or a single-stage oxidation method using ahypochlorite. However, an amount of carboxyl groups in thepolycarboxylic acid obtained by the method disclosed in this gazettedoes not exceed two on average per monosaccharide unit, excluding aspecial example. As the special example wherein the content of carboxylgroups exceeds 2 per monosaccharide unit, Example 78 discloses theexample of previously producing a monocarboxylated corn starch andfurther oxidizing the monocarboxylated corn starch and Examples 79 and80 disclose the example using sodium arginate as polysaccharides.However, in these Examples, there is no description about the molecularweight of the resulting polycarboxylic acid and, therefore, theseExamples do not suggest a polycarboxylic acid having a structure whereinboth content of carboxyl groups and molecular weight are clearlydefined. In the production method described in these Examples, a largeamount of acetic acid is used when the formed dialdehyde derivative isoxidized with sodium hypochlorite to form a dicarboxylic acid. However,it is well known that, under these acidic conditions, polysaccharidesare hydrolyzed and the molecular weight is drastically lowered. Besides,this method has a problem that the production process becomescomplicated because the monocarboxylated starch is once produced andfurther oxidized. On the other hand, when arginic acid is used as a rawmaterial, the method has an economical problem. JP-A-60-226502 disclosesa method of oxidizing polysaccharides using a hypochlorite as anoxidizing agent under controlled reaction conditions to obtain apolycarboxylic acid. The polycarboxylic acid obtained by this method hassufficiently high molecular weight, but the content of the carboxyl unitis 81% at most and the content of carboxyl groups is less than 2 onaverage per monosaccharide unit. Therefore, it is not a polycarboxylicacid having a structure which simultaneously satisfies the content ofcarboxyl groups and the molecular weight.

JP-A-62-247837 discloses a method of oxidizing polysaccharides using ametal catalyst such as Pd in combination with an accelerator such as Bito obtain a polycarboxylic acid. However, this method is characterizedby oxidizing the reducing terminal end of polysaccharides and thecontent of carboxyl groups does not also exceed 2 on average permonosaccharide unit.

JP-A-4-175301 discloses a method of oxidizing polysaccharides in thepresence of a hypobromite or a hypoiodite. This gazette does notdisclose data which suggest the structure of a polycarboxylic acid as aproduct. However, since there is a description "it is understood in thisspecification that the term `dicarboxy polysaccharides or polycarboxysaccharides` used herein mean polysaccharides wherein almost all ofC2-C3 diol functional groups are respectively converted into twocarboxyl groups by means of ring opening" in the specification, theamount of carboxyl groups contained in the polycarboxylic acid of thisgazette is considered to be 2 at most as an average value permonosaccharide unit.

JP-A-4-233901 discloses a method of oxidizing an enzyme hydrolysate ofstarch or dextrin with a hypochlorite or a periodate. In this gazette,there is no description about a structure of a polycarboxylic acidobtained after the oxidation reaction, particularly content of carboxylgroups. Also, a clear measurement example about the molecular weight isnot disclosed. Even if the molecular weight is calculated based on adistribution of a glucoside unit disclosed in Example 5 assuming that adecrease in molecular weight does not arise at the time of the followingoxidation reaction and three carboxyl groups are introduced per unitafter the maximum reaction, a weight-average molecular weight is 915 atmost. When using, as a raw material, starch or dextrin from which ahigher molecular weight material may be obtained, as shown inComparative Example 1, a Ca ion sequestration capacity of the resultingpolycarboxylic acid is still low, e.g. 200 and 225, which shows that thecontent of carboxyl groups in the oxidation reaction product is low.

JP-A-08-500626 discloses a method of oxidizing potato starch suspendedin carbon tetrachloride with dinitrogen tetraoxide, followed byoxidation with oxygen. The carboxyl group content of the polycarboxylicacid obtained by this method is very low, e.g. about 1 in average numberof carboxyl groups per glucose unit.

As described above, those wherein the content of carboxyl groups exceeds2 per anhydrous glucose unit and the weight-average molecular weight isnot less than 2000 are not known with respect to a polycarboxylic acidobtained by oxidizing polysaccharides containing an anhydrous glucose asa constituent unit, or a salt thereof.

DISCLOSURE OF THE INVENTION

The object of the present invention of the present invention is toprovide a method of producing a polycarboxylic acid or a salt thereofhaving a high carboxylic group content and a large molecular weight frompolysaccharides containing an anhydrous glucose as a constituent unit,and a polycarboxylic acid or a salt thereof obtained by the method.

The present inventors have intensively studied to solve the aboveproblems. As a result, the present invention has been accomplished.

That is, according to the present invention, there is provided apolycarboxylic acid or a salt thereof derived from polysaccharidescontaining an anhydrous glucose as a constituent unit, characterized inthat the amount of an alkali required to perform neutralizationtitration of an acid type polycarboxylic acid is not less than 435 mg interms of sodium hydroxide per 1 g of said polycarboxylic acid and theweight-average molecular weight of said polycarboxylic acid is not lessthan 2000.

According to the present invention, there is also provided a method forproducing a polycarboxylic acid or a salt thereof, wherein the amount ofan alkali required to perform neutralization titration of an acid typepolycarboxylic acid is not less than 435 mg in terms of sodium hydroxideper 1 g of the polycarboxylic acid and the weight average molecularweight of said polycarboxylic acid is not less than 2000, characterizedin that a polysaccharide containing an anhydrous glucose as aconstituent unit is oxidized in the presence of a transition metalcatalyst using an oxidizing agent.

Since the structure of the polycarboxylic acid of the present inventionis complicated, it is difficult to show a clear structural formula. Thepolycarboxylic acid obtained by using starch as a raw material isrepresented by following general formula (1) and, assuming by means ofan amount of sodium hydroxide required to neutralization titration, itcan be said to be a polycarboxylic acid having at least 2.1 carboxylgroups on average per anhydrous glucose unit. ##STR1## wherein X is H ora salt-forming cation and (3P+2m+n)/(l+m+n) is from 2.1 to 3.0.

The reason is as follows. That is, in case of a polycarboxylic acidhaving two carboxylic groups per anhydrous glucose, wherein a c2-C3 bondof an anhydrous glucose is formed by oxidation ring opening (i.e.dicarboxyl polysaccharide of the above structural formula wherein P is 0and n is 0), the amount of sodium hydroxide required to neutralizationtitration is 417 mg at most per 1 g of an acid type polycarboxylic acid.

In the salt of the polycarboxylic acid of the present invention,examples of the salt include salts of alkaline metals such as Na, K, Li,etc. and salts of amines such as ammonia, alkylamine, alkanolamine, etc.Among them, Na salt or K salt is preferable in view of ease ofproducing. A part of carboxyl groups may be converted into a salt.

In the present invention, as the method of determining the content ofcarboxyl groups, a neutralization titration method can be used asdescribed above. When an acid type polycarboxylic acid must be obtainedin case of the neutralization titration, it is carried out by using themethod of treating with a cation exchange resin described hereinafter.The method of determining the molecular weight is carried out using theGPC method using a standard polyacrylic acid as described hereinafter.

The polysaccharides used for obtaining the polycarboxylic acid of thepresent invention or salt thereof may be any polysaccharides containingan anhydrous glucose as a constituent unit, and are not specificallylimited. Examples thereof include starch, dextrin, cellulose or ahydrolysate thereof, or amylose or amylopectin obtained by fractionatingstarch. These polysaccharides are used in combination thereof. Also, anorigin of these polysaccharides is not specifically limited. In case ofstarch, for example, corn starch, wheat starch, rice starch, tapiocastarch, etc. may be used. In case of cellulose, for example, celluloseobtained from conifer tree, broad leaf tree, cotton, etc. may be used.Among these polysaccharides, starch or cellulose is preferably used inview of the availability or economical efficiency. Particularlypreferable one is starch.

The polycarboxylic acid of the present invention or salt thereof isobtained by oxidizing polysaccharides containing an anhydrous glucoseusing a combination of a specific catalyst and a specific oxidizingagent. That is, the polycarboxylic acid or salt thereof can be producedby adding an oxidizing agent with maintaining the pH of the solutionwithin a fixed range.

Examples of the transition metal catalyst used in the present inventionare Ru, Os, Rh, Ir, Pt, Pd, etc., preferably ruthenium catalyst orosmium catalyst. These transition metal catalysts may be used in theform of salts such as chloride, sulfide, oxide, etc. or used as they areor after carrying them with carriers such as carbon, alumina, etc. Anamount of these metal catalysts used is from 0.05 to 10% by mol,preferably from 0.1 to 7% by mol, more preferably from 0.5 to 5% by mol,per anhydrous glucose unit of polysaccharides.

Examples of the oxidizing agent used in the present invention includehypohalite (e.g. Na salt, K salt, Ca salt, Mg salt, etc.), bleachingpowder, perhalite, persulfate, peracetate and the like. Preferably,bleaching powder or hypohalite is used and use of hypohalite isparticularly preferable. An amount of these oxidizing agents used variesdepending on the content of carboxyl groups in the polycarboxylic acidto be finally obtained, but is normally from 3 to 12 mol, preferablyfrom 4 to 9 mol, per mol of an anhydrous glucose unit ofpolysaccharides. The reaction is performed by adding these oxidizingagents into a dispersion of polysaccharides and a transition metalcatalyst in water. A method of adding the oxidizing agent is notspecifically limited, but the oxidizing agent is normally addedcontinuously or added by several portions. The reaction time is notspecifically limited, but is normally from 2 to 24 hours, preferablyfrom 3 to 18 hours. The reaction temperature is not specificallylimited, but is normally from 5 to 50° C., preferably from 10 to 40° C.

The pH of the solution during the reaction is maintained within therange from 6 to 13, preferably from 7 to 12. Adjustment of the pH can beperformed by using hydroxides of alkaline metals, such as sodiumhydroxide, potassium hydroxide, lithium hydroxide, etc.; or amines suchas ammonia, alkylamine, alkanolamine, etc. Preferably, sodium hydroxideor potassium hydroxide is used.

Under the above reaction conditions, cleavage of a glucoside bond of abackbone chain of polysaccharides does not arise, comparatively easily,and a hydroxyl group of polysaccharides is oxidized in good efficiency.Therefore, it is possible to obtain a desired polycarboxylic acidwherein an amount of sodium hydroxide required to perform neutralizationtitration of an acid type polycarboxylic acid is not less than 435 mgper 1 g of the polycarboxylic acid and a weight-average molecular weightis not less than 2000, or a salt thereof. The upper limit of themolecular weight of the polycarboxylic acid is about 50000 inweight-average molecular weight. This polycarboxylic acid or saltthereof is suitably used as a builder for detergent or a scum inhibitorbecause a chelate force to a polyvalent cation such as Ca, Mg, etc. ishigh as a result of high molecular weight and high content of carboxylgroups.

In the polycarboxylic acid produced by the oxidation of polysaccharidesof the present invention, the amount of neutralization titration usingsodium hydroxide (NaOH) and molecular weight vary depending on the kindof raw polysaccharides used, kind of an oxidizing agent, kind of acatalyst and reaction conditions. In case of the present invention, theamount of neutralization titration is not less than 435 mg/g,particularly from 486 to 582 mg/g, and the molecular weight is not lessthan 2000, preferably from 2,500 to 15,000, in weight-average molecularweight. In the present invention, it is particularly preferable that thepolycarboxylic acid is produced by oxidizing starch as the rawpolysaccharides with a hypochlorite or a bleaching powder in thepresence of a ruthenium catalyst.

In the present invention, the oxidation reaction product obtained by theabove oxidation reaction is preferably treated with a reducing agent. Acolor tone of the oxidation reaction product can be improved by thistreatment using the reducing agent. The oxidation reaction product showsa black or brown color and is inferior in color tone, but the color tonecan be improved to form a color tone (e.g. blue color, yellow color,etc.) by subjecting to a reducing agent treatment.

The reducing agent may be any one whose redox potential is lower thanthat of the oxidizing agent used, and preferable examples thereofinclude sulfite (e.g. sodium salt, potassium salt, ammonium salt, etc.),ditionite (e.g. sodium salt, potassium salt, ammonium salt, etc.) andhydrogen peroxide. An amount of these reducing agent used variesdepending on the amount of the oxidizing agent remained in the reactionsystem after the completion of the oxidation reaction, but is generallyfrom 0.3 to 10 mol, preferably from 0.5 to 5 mol, per mol of theresidual oxidizing agent.

Next, the method for the measurement of the neutralization titrationamount (hereinafter referred to as "carboxyl group content A",sometimes) and molecular weight in the polycarboxylic acid will bedescribed.

[Measurement of carboxyl group content A]

After precisely weighing about 0.2 g (absolute dry weight) of an acidtype polycarboxylic acid, the acid type polycarboxylic acid is chargedin a conical beaker having a volume of 200 ml and about 50 ml ofdeionized water is added to dissolve the acid type polycarboxylic acid.Using phenolphthalein as an indicator and a 1/10 N standard sodiumhydroxide solution, titration of the resulting solution is performed anda carboxyl group content A is indicated as an amount (mg) of sodiumhydroxide required to neutralize 1 g of the acid type polycarboxylicacid.

When it is apparent that a part or all of the polycarboxylic acid whosecarboxyl group content A is measured is in the form of a salt, thepolycarboxylic acid is converted into an acid type polycarboxylic acidby the following method. That is, it is converted into the acid typepolycarboxylic acid by preparing an aqueous solution (about 1% byweight) of the polycarboxylic acid, passing the aqueous solution througha column packed with a cation exchanged resin (DOWEX 50W-X8), followedby cation exchange. The cation exchange resin is used in an amount of 10ml per 1 g of an aqueous solution (1% by weight) of the polycarboxylate.The acid type polycarboxylic acid can be obtained in the form of apowder by freeze-drying the eluent or drying it under reduced pressure(40° C. or less).

[Measurement of molecular weight by means of GPC]

About 5 mg of the same polycarboxylic acid powder as that used in theneutralization titration is dissolved in 5 ml of a phosphate buffersolution (pH 7, concentration: 0.1 mol/liter) containing NaCl having aconcentration of 0.3 mol/liter and the molecular weight is determined bymeans of GPC under the following conditions and was indicated by theweight-average molecular weight.

Column to be used: manufactured by Toso Co., Ltd., G-4000PW+G-2500PW

Eluent: phosphate buffer (concentration: 0.1 mol/liter) containing NaClhaving a concentration of 0.3 mol/liter, pH7

Eluation rate: 0.5 ml/min.

Column temperature: 40° C.

Amount of sample to be charged: 200 μl

Calibration curve: standard Na polyacrylate manufactured by PolyscienceCo. (weight-average molecular weight: 2100, 5000, 20000, 35000 and165300) was used.

EXAMPLES

The following Examples will further illustrate the present invention indetail. In the followings, percentages are by weight unless otherwisestated.

Example 1

In a separable flask (volume: 500 ml) equipped with a stirrer, adropping funnel for oxidizing agent, a dropping inlet for aqueous sodiumhydroxide solution, a pH electrode and a thermometer, 10 g (absolute dryweight) of corn starch, 100 g of deionized water and 0.49 g (3% by molper anhydrous glucose unit of starch) of RuCl₃ ·nH₂ O (Ru content: 38%by weight) were charged after weighing. The mixture was cooled with awater bath of 20° C. and, at the time when the inner temperature reachesabout 20° C., 230 g (6 mol per mol of an anhydrous glucose unit) of anaqueous solution (12 % by weight) of Na hypochlorite was added over 3hours. During the addition, an aqueous 2 N sodium hydroxide solution wasadded using a pH-stat to control the pH in the system to 9.

After the completion of the addition of Na hypochlorite, stirring wascontinued for additional two hours, the reaction mixed solution wasslowly poured into about 1 liter of ethanol to precipitate the reactionproduct. The resulting precipitate was dissolved in 150 ml of deionizedwater, purified by subjecting to diffusion dialysis using deionizedwater for 5 days and then freeze-dried to obtain 7.5 g of a powder of Napolycarboxylate.

With respect to the resulting Na polycarboxylate, the carboxyl groupcontent A obtained by the above method was 521 mg and the weight-averagemolecular weight was 5800.

A Ca ion sequestration capacity (CEC) determined by the following methodwas 410 mg/g.

[Measurement of Ca ion sequestration capacity (CEC)]

A Ca solution having a concentration of 100 ppm in terms of CaCO₃ wasprepared by diluting a standard CaCl₂ solution having a concentration of0.1 mol/liter (manufactured by Orion Research Co.) in a dilution of 100.On the other hand, the polycarboxylic acid to be tested was dissolved indeionized water to prepare an aqueous solution wherein a concentrationof Na polycarboxylate is precisely 1% by weight, which was taken as atest solution.

After weighing 100 ml of the Ca solution, 2 ml of a KCl solution havinga concentration of 4 mol/liter was added and the pH of the solution wasadjusted to 10 by using 1/10 N NaOH. An initial concentration of the Caion was measured with stirring this aqueous solution using a Ca ionelectrode. After 2 ml of the test solution was precisely added and thepH was adjusted again to 10, a Ca ion concentration was measured byusing a Ca ion electrode. The amount obtained by subtracting the Ca ionconcentration after the addition of the test solution from the initialconcentration of the Ca ion was indicated as an amount of Ca ionchelated. The amount of the Ca ion chelated by 1 g of Na polycarboxylatewas indicated as an amount (mg) of CaCo₃.

Example 2

In the same separable flask (volume: 500 ml) as that shown in Example 1,10 g (absolute dry weight) of corn starch, 50 g of deionized water and0.16 g (1% by mol per anhydrous glucose unit of starch) of RuCl₃ ·nH₂ O(Ru content: 38% by weight) were charged after weighing. The mixture wascooled with a water bath of 20° C. and, at the time when the innertemperature reaches about 20° C., 192 g (5 mol per mol of an anhydrousglucose unit) of an aqueous Na hypochlorite solution (12% by weight) wasadded over 5 hours. During the addition, an aqueous 2 N sodium hydroxidesolution was added using a pH-stat to control the pH in the system to10.

After the completion of the addition of Na hypochlorite, stirring wascontinued for additional two hours and then 7.8 g of Na polycarboxylatewas obtained according to the same manner as that described in Example1.

The carboxyl group content A in the resulting Na polycarboxylate was 489mg, the weight-average molecular weight was 8100, and CEC was 402 mg/g.

Comparative Example 1

3 g of Na polycarboxylate obtained in Example 2 was dissolved in 50 g ofdeionized water and, after the pH was adjusted to 3.0 by using 1 Nhydrochloric acid, the solution was hydrolyzed with heating in a hotwater bath at 80° C. under stirring using a magnetic stirrer for 2hours. After cooling to room temperature, the pH was adjusted to 10 byusing 1 N NaOH and the resultant was poured into 500 ml of ethanol. Theresulting precipitate was dissolved in 30 ml of deionized water,subjected to diffusion dialysis using deionized water for 5 day and thenfreeze-dried to obtain 1.1 g of hydrolyzed Na polycarbonate.

The carboxyl group content A in this hydrolyzed Na polycarboxylate was486 mg, but the weight-average molecular weight is reduced to 1200 andCEC was also reduced to 280 mg/g.

Example 3

According to the same manner as that in Example 1 except for using 0.47g (3% by mol per an anhydrous glucose unit of starch) of osmiumtetraoxide in place of RuCl₃ ·nH₂ O (Ru content: 38% by weight), 6.8 gof Na polycarboxylate was obtained. The carboxyl group content A of theresulting Na polycarboxylate was 458 mg and the weight-average molecularweight was 4200. CEC was 340 mg/g.

Example 4

According to the same manner as that in Example 1 except for using 10 ga cellulose powder (reagent) in place of corn starch, 8.2 g of Napolycarboxylate was obtained. The carboxyl group content A of theresulting Na polycarboxylate was 440 mg and the weight-average molecularweight was 6200. CEC was 365 mg/g.

Examples 5 to 9

According to the same manner as that described in Example 1 except forvarying the reaction conditions such as, kind of polysaccharides, kindof a catalyst, amount of Na hypochlorite (NaClO) and pH during thereaction, Na polycarboxylate was obtained, respectively.

The Ru content of a carbon-carrying Ru shown in Table 1 is 5% by weight,and each of the amount of the catalyst and amount of NaClO used is anamount per anhydrous glucose unit of polysaccharides.

                                      TABLE 1                                     __________________________________________________________________________                  Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                       Item 5 6 7 8 9                                                              __________________________________________________________________________    Reaction                                                                             Kind of                                                                              Corn Wheat                                                                              Amylopec                                                                           Corn Corn                                          conditions polysacchar starch starch tin starch starch                         ides                                                                          Kind of RuCl.sub.3 RuCl.sub.3 Carbon- RuCl.sub.3 RuO.sub.2                    catalyst   carrying                                                              Ru                                                                         Catalyst 5 7 1 0.5 0.5                                                        amount                                                                        (% by mol)                                                                    NaClO (mol) 6 9 6 5 5                                                         pH 11 10 8 7 9                                                                Temperature 20 20 20 30 20                                                    (° C.)                                                              Yield (g)     7.8  7.4  8.3  8.1  7.9                                         Property                                                                             Carboxyl                                                                             551  558  514  488  451                                            group                                                                         content A                                                                     (mg)                                                                          Weight- 4300 2800 6200 3800 12500                                             average                                                                       molecular                                                                     weight                                                                       Performance CEC 425 408 412 397 389                                            (mgCaCO.sub.3 /g)                                                          __________________________________________________________________________

Example 10

In the same separable flask (volume: 500 ml) as that shown in Example 1,10 g (absolute dry weight) of corn starch and 90 g of deionized waterwere charged.

Thereafter, the separable flask was put in a constant temperature bathat 20° C. and corn starch was dispersed by stirring. Then, 0.49 g (3% bymol per an anhydrous glucose unit of starch) of RuCl₃ ·nH₂ O (Rucontent: 38% by weight) was added, followed by stirring. At the timewhen the inner temperature reaches about 20° C., 213 g (6 mol per mol ofan anhydrous glucose unit) of an aqueous Na hypochlorite solution (13%by weight) was added dropwise for 6 hours, continuously. During thedropwise addition, an aqueous 2 N sodium hydroxide solution was addeddropwise using a pH-stat to control the pH in the system to 8.

After the completion of the dropwise addition of the aqueous sodiumhypochlorite solution, the concentration of the residual sodiumhypochlorite was measured. As a result, it was 1% by weight. Then,sodium sulfite was added in a one-fold molar amount based on theresidual sodium hypochlorite and then dissolved by stirring. After thedissolution, the solution was allowed to stand to form a precipitate.After the formed precipitate was removed by filtration (pore diameter offilter: 0.1 μm), the filtrate was slowly poured into a two-fold amountof ethanol to precipitate the reaction product. The resultingprecipitate was dissolved in 100 ml of deionized water, purified bysubjecting to diffusion dialysis using deionized water for 5 days andthen freeze-dried to obtain 7.2 g of a powder of sodium polycarboxylate.

The carboxyl group content A of the resulting sodium polycarboxylate was500 mg and the weight-average molecular weight was 4000. The color tonewas pale blue. According to the same manner as that described in theabove test except for using no sodium sulfite, 7.5 g of a powder ofsodium polycarboxylate was obtained. The carboxyl group content A of theobtained by the above method was 500 mg and the weight-average molecularweight was 4000. The color tone of the powder was black.

Example 11

The same manner as that described in Example 10 was repeated, exceptthat the amount of sodium sulfite was changed. The results are shown inTable 2.

                  TABLE 2                                                         ______________________________________                                        Sodium sulfite                                                                           0.5          2        5                                              (mol per mol of                                                               oxidizing                                                                     agent)                                                                        Carboxyl group 500 500 500                                                    content A (mg)                                                                Weight-average 4000 4000 4000                                                 molecular                                                                     weight                                                                        Color tone of Bluish green Nearly Nearly                                      powder  colorless colorless                                                 ______________________________________                                    

Example 12

According to the same manner as that described in Example 10 except forchanging the amount of RuCl₃ ·nH₂ O (Ru content: 38% by weight) to 0.05g (0.3% by mol per anhydrous glucose unit of starch), the reaction wasperformed.

After the completion of the dropwise addition of the aqueous sodiumhypochlorite solution, the concentration of the residual sodiumhypochlorite was 1.3%. Then, sodium sulfite was added in a one-foldmolar amount based on the residual sodium hypochlorite and thendissolved by stirring. After the dissolution, the solution was allowedto stand to form a precipitate. After the formed precipitate was removedby filtration (pore diameter of filter: 0.1 μm), the filtrate was slowlypoured into a two-fold amount of ethanol to precipitate the reactionproduct. The resulting precipitate was dissolved in 100 ml of deionizedwater, purified by subjecting to diffusion dialysis using deionizedwater for 5 days and then freeze-dried to obtain 5.2 g of a powder ofsodium polycarboxylate.

The carboxyl group content A of the resulting sodium polycarboxylate was450 mg and the weight-average molecular weight was 2500. The color tonewas pale yellow.

Example 13

According to the same manner as that described in Example 10 except forchanging the amount of RuCl₃ ·nH₂ O (Ru content: 38% by weight) to 1.6 g(10% by mol per anhydrous glucose unit of starch), the reaction wasperformed.

After the completion of the dropwise addition of the aqueous sodiumhypochlorite solution, the concentration of the residual sodiumhypochlorite was 1.2%. Then, sodium sulfite was added in a two-foldmolar amount based on the residual sodium hypochlorite and thendissolved by stirring.

After the dissolution, the solution was allowed to stand to form aprecipitate. After the formed precipitate was removed by filtration(pore diameter of filter: 0.1 μm), the resulting precipitate waspurified by using an ultrafiltration device and then freeze-dried toobtain 5.2 g of a powder of sodium polycarboxylate.

The carboxyl group content A of the resulting sodium polycarboxylate was520 mg and the weight-average molecular weight was 4000. The color tonewas pale blue.

Example 14

In the same separable flask (volume: 500 ml) as that shown in Example 1,10 g (absolute dry weight) of corn starch and 90 g of deionized waterwere charged.

Thereafter, the separable flask was put in a constant temperature bathat 20° C. and corn starch was dispersed by stirring. Then, 0.16 g (1% bymol per an anhydrous glucose unit of starch) of RuCl₃ ·nH₂ O (Rucontent: 38% by weight) was added, followed by stirring. At the timewhen the inner temperature reaches about 20° C., 213 g (6 mol per mol ofan anhydrous glucose unit) of an aqueous Na hypochlorite solution (13%by weight) was added dropwise for 3 hours, continuously. During thedropwise addition, an aqueous 2 N sodium hydroxide solution was addeddropwise using a pH-stat to control the pH in the system to 10.

After the completion of the dropwise addition of the aqueous sodiumhypochlorite solution, the concentration of the residual sodiumhypochlorite was measured. As a result, it was 0.5% by weight. Then, areducing agent shown in Table 3 was added in a five-fold molar amountbased on the residual sodium hypochlorite and then dissolved bystirring. After the dissolution, the solution was allowed to stand toform a precipitate. After the formed precipitate was removed byfiltration (pore diameter of filter: 0.1 μm), the filtrate was slowlypoured into a two-fold amount of ethanol to precipitate the reactionproduct. The resulting precipitate was dissolved in 100 ml of deionizedwater, purified by subjecting to diffusion dialysis using deionizedwater for 5 days and then freeze-dried to obtain a powder of sodiumpolycarboxylate. The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                                   Na dithionite                                        Reducing agent Sodium sulfite (Hydrosulfite)                                ______________________________________                                        Carboxyl group content                                                                         460        460                                                 A (mg)                                                                        Weight-average 3000 3000                                                      molecular weight                                                              Color tone of powder Pale blue Pale yellow                                  ______________________________________                                    

Example 15

According to the same manner as that described in Example 10 except forchanging the reaction conditions as shown in Table 4, the reaction wasperformed. After the completion of the dropwise addition of an aqueoussodium hypochlorite solution, the amount of the residual sodiumhypochlorite was measured and then a sodium sulfite was added in anamount of 2 mol per mol of the residual sodium hypochlorite. The resultsare shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    Test No.                                                                             1   2   3   4   5   6     7    8                                       __________________________________________________________________________    Kind of                                                                              Corn                                                                              Tapioca                                                                           Potato                                                                            Wheat                                                                             Soluble                                                                           Amylope                                                                             Cellulose                                                                          Corn                                      polysaccharides starch starch starch starch starch ctin powder starch                                              Kind of RuCl.sub.3 RuCl.sub.3                                                RuCl.sub.3 RuCl.sub.3 RuCl.sub.3                                              Carbon- RuCl.sub.3 None                   catalyst      carrying Ru                                                     Catalyst 3 3 5 1 3 0.5 3 0                                                    amount (% by                                                                  mol)*                                                                         Sodium 3 6 6 12 5 6 6 6                                                       hypochlorite                                                                  amount                                                                        (mol)**                                                                       Time of 3 6 18 6 5 1.5 3 3                                                    sodium                                                                        hypochlorite                                                                  to be added                                                                   (hour)                                                                        Reaction 20 20 20 40 5 20 20 20                                               temperature                                                                   (° C.)                                                                 Reaction pH 9 12 6 9 8 8 10 9                                                 Carboxyl 513 517 537 517 501 450 460 312                                      group                                                                         content A                                                                     (mg)                                                                          Weight- 3300 4200 6600 2700 3500 3600 5500 5000                               average                                                                       molecular                                                                     weight                                                                        Color tone of Pale Pale Pale Pale Pale Pale Pale Colorless                    powder blue blue yellow blue blue yellow blue                               __________________________________________________________________________     *per anhydrous glucose unit                                                   **per mol of anhydrous glucose unit                                      

Example 16

According to the same manner as that in Example 10 except for using 0.47g (3% by mol per an anhydrous glucose unit of starch) of osmiumtetraoxide in place of RuCl₃ ·nH₂ O, 6 g of sodium polycarboxylate wasobtained. The carboxyl group content A of the resulting sodiumpolycarboxylate was 450 mg. The weight-average molecular weight was 3800and the color tone of the powder was pale yellow.

EFFECT OF THE INVENTION

According to the present invention, there can be obtained apolycarboxylic acid having high carboxyl group content and highmolecular weight, or a salt thereof, in good yield.

The polycarboxylic acid of the present invention or salt thereof can beused as a dispersant, a chelating agent or a flocculant. Since thecarboxyl group content is high and a chelate force to a polyhydriccation such as Ca, Mg, etc. is large, the polycarboxylic acid or saltthereof is particularly suitable as a builder for detergent or a scuminhibitor.

We claim:
 1. A method for producing a polycarboxylic acid or a saltthereof, wherein the polycarboxylic acid contains, in its acid form,carboxylic acid groups in a number requiring an amount of alkali forneutralization of its acid form not less than 435 mg in terms of sodiumhydroxide per 1 g of the polycarboxylic acid and the weight-averagemolecular weight of said polycarboxylic acid is not less than 2000,comprising oxidizing a polysaccharide containing anhydrous glucose as aconstituent unit in the presence of a transition metal catalyst, usingan oxidizing agent selected from the group consisting of hypohalites,bleaching powder, perhalites, persulfates and peracetates, saidtransition metal catalyst containing at least one member selected fromthe group consisting of Ru, Os, Tr, Pt, Pd and oxides and salts thereof.2. A method according to claim 1 wherein said transition metal catalystcontains at least one member selected from the group consisting of Ruand Os and oxides and salts thereof.